1. Field of the Invention
The present invention relates to a process for producing rigid polyurethane foam.
2. Description of the Background
Rigid polyurethane foams are usually produced by agitating and mixing instantaneously a polyisocyanate and a polyol containing a blowing agent (a chlorofluorocarbon, i.e., CFC which is a halogenated hydrocarbon, and water), a catalyst, a silicone foam stabilizer, and other optional additives, and foaming the mixture. Rigid polyurethanes are widely used for heat insulators in fields of application requiring hot insulation or cold insulation such as building materials, boards, electric refrigerators and manufacturing plants, for example, because of their light weight and superior heat-insulating properties. Spray foaming thereof is advantageous in applicability to in-place foaming.
A rigid polyurethane foam is an urethane resin which has a structure having independent cells, namely closed cells, where a CFC compound and water used as blowing agents are enclosed in individual cells as CFC gas having low heat conductivity and carbon dioxide gas having a relatively high heat conductivity resulting from the reaction of water with an isocyanate. Accordingly, for the purpose of improving heat insulation properties, conventional formulations generally employed have been such that, as the blowing agent, water is used in a minimum amount and a CFC compound is used in a larger amount: for example, 0 to 1.5 parts by weight of water, and 35 to 60 parts by weight of a CFC compound based on 100 parts by weight of the polyol. Catalysts employed generally include triethylenediamine, tetramethylhexamethylenediamine and dimethylcyclohexylamine, for example. With such a formulation, useful rigid polyurethane foams have been produced industrially.
As mentioned above, the heat insulation properties of conventional rigid polyurethane foams have been improved by increasing the amount of the CFC compounds used to lower the thermal conductivity.
Recently, however, it has been suggested that CFC compounds may destroy the ozone layer of the earth. Consequently, the regulation of CFC use has started throughout the world to virtually eliminate the use of CFCs by the end of this century. On the other hand, the use of a conventionally large amount of the HCFC (hydrochlorofluorocarbon) alternatives for CFCS, which exhibit a low ozone destruction coefficient, is not desirable from the stand point of production cost. Thus, a need clearly exists for a technique which can minimize the use of CFCs in the production and use of rigid polyurethane foams.
For this purpose, formulations have been, and are being, investigated in which the amount of the CFC is decreased, and instead, the amount of water is increased as the blowing agent. However, this poses several technical problems. For example, the use of a larger amount of water as the blowing agent generates carbon dioxide gas which fills the cell in large amount in place of the CFC gas, which lowers the CFC concentration in the cells and adversely affects the heat conductivity. The increase of the amount of water in the formulation also causes an increase in formation of urea linkages having higher crystallinity by reaction of the isocyanate with water, in addition to the normal urethane-forming reaction of a polyol with an isocyanate. Thus, the foam becomes brittle, or becomes worse in friability.
The deterioration in friability is especially remarkable in the case where diphenylmethane-4,4'-diisocyanate and/or a polymeric isocyanate thereof is used as the isocyanate, and in the case where toluene diisocyanate and/or a mixture thereof with the prepolymer thereof is used. The deterioration in friability causes many problems such as a decrease in strength of the foam, and a decrease of adhesiveness between a foam material and a surface material, for example, all which are great technical problems. In the case of spray-foamed rigid polyurethanes produced by use of a conventional catalyst, such as triethylenediamine or tetramethylhexamethylenediamine, for example, another problem is involved in that adhesiveness to a plate material, particularly an iron plate, a gypsum board, for example, becomes significantly lower when foamed at a low temperature, especially during winter.
Since the primary object is to decrease the amount of CFC which is used, the deterioration in heat conductivity appears to be unavoidable to some extent. However, the deterioration in friability is a serious problem relating to the usefulness of the products.
Thus, a need continues to exist for a rigid polyurethane foam which has superior properties even when using a decreased amount of CFC.